COMBINING ABILITY IN ELEVEN MAIZE (Zea mays L.) FEW TASSEL BRANCH MUTANTS (fbr) UNDER OPTIMUM AND DROUGHT STRESS ENVIRONMENTS
Maize (Zea mays L.) is an important cereal crop and primary source of calories in Zimbabwe. Generally two groups of farmers are involved in maize production, the large scale commercial sector (LSC) and the communal farming sector (CFS) categorized based on production systems and scales of operation. Production is dominated by the CFS although yield levels are lower than the LSC. Production in highly stress prone environments with no or limited access to resources to mitigate the stresses has been responsible for the yield gap. Although the tassel is an essential reproductive organ, it often reduces yield either physiologically by competing with the ear or physically by shading effect. The competition effects and shading effects are more pronounced under stress and high density respectively. By reducing the size of the tassel, breeders can reduce the competition effects of tassels especially under drought stress and contribute to higher yields. Eleven CIMMYT few tassel branch (fbr) mutants and three testers, CML442 TAS, CML442 and CML395 were crossed in a 11 x 3 line x tester design at CIMMYT- Zimbabwe in 2008/09. The testcrosses together with checks were evaluated under one optimum environment and two drought environments in a randomized complete block design with two replications during winter 2009. The objectives of the study were to estimate general and combining ability effects of the eleven fbr lines and assess the relationship between grain yield and tassel traits. Seven tassel traits, tassel branch angle, tassel size, total tassel length, central spike length, branch zone length, branch length and tassel weight were recorded. Results revealed significant (P<0.05) differences among hybrids and lines for grain yield across all environments and under the optimum environment but not under drought environments. There were no significant differences among testers for grain yield under all management levels. Significant differences (P<0.05) were also observed among hybrids, lines and testers for all tassel traits across all management levels. L11 showed consistently good GCA for grain yield under both drought and optimum environments, while L2, L6 and L7 showed consistently poor GCA. Hybrids C10-2 and C6-2 showed consistently good SCA for grain yield under both optimum and drought environments while C8-2, C10-3, C3-2, C9-2 and C1-1 showed consistently poor SCA. Lines and testers with good GCA for grain yield showed negative GCA effects for tassel branch angle and anthesis date but positive GCA effects for total tassel length, central spike length, branch zone length, branch length and tassel weight. Similarly hybrids with good combining ability for grain yield showed negative SCA effects for tassel branch angle and anthesis date but positive GCA effects for total tassel length, central spike length, branch zone length, branch length and tassel weight. These hybrids were constituted by parents with an unrelated genetic background. High broad sense heritabilities ranging from 37% (branch zone length) under drought environments to 86% (central spike length) under the optimum environment were found for all tassel traits. Grain yield was negatively correlated with tassel branch angle and anthesis date but positively correlated with total tassel length, central spike length, branch zone length, branch length and tassel weight. Given the high heritability of tassel traits, high correlation with grain yield, easiness and cheapness in measurement they are good candidates for use as secondary traits.
Corn--Breeding--Zambia , Maize (Zea mays L.) --Zambia